As others have said, those four extra quarks are higher energy versions of the up and down. If you take a look at the standard model you will find that the very same works for electrons, which are the “base version” of muons and tauons. Only with their neutrinos it is a bit different, they can even turn into each other. In general the higher variants decay rather quickly into the base forms, hence why they are rarely around.
So in short: there are two base quarks (up, down), and two basic leptons (electron, e-neutrino). Each has three “variants”, plus all have anti-particles. Stuff prefers to be basic.
Plus the force carriers, which also somewhat group together under electro-weak unification/the Higgs mechanism (except gluons, and hypothetical gravitons… yet). But that’s another story.
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